Design of Light-Responsive Protein Assemblies

The majority of dynamic biological phenomena, from signaling cascades to structural rearrangement, involve protein-based assemblies that respond to external stimuli. Such natural protein-based machines and materials have inspired extensive efforts in protein design and engineering. Light represents a particularly attractive stimulus to control protein self-assembly as it is non-invasive, spatially and temporally controllable and does not rely on diffusive processes like chemical stimuli. However, to the best of our knowledge, there are no designed protein assemblies with a built-in light trigger, and most proteins that are engineered to be light-responsive involve fusion to a naturally light-sensitive protein domain. One of the overarching goals of our research program is to develop chemical strategies to design protein assemblies using chemically tunable interactions. Here, we set out to exploit light-controllable, supramolecular host-guest interactions to mediate the formation of artificial protein assemblies, with the ultimate goal of developing novel functional, responsive and self-healing biomaterials. Toward this end, we used the well-characterized β-cyclodextrin (β-CD) as a molecular host and azobenzene as the molecular guest whose UV-visible light-triggered <i>trans-cis</i> isomerization effects the association and dissociation of the host-guest complex. A <i>C</i>₄ symmetric, tetrameric protein, RhuA, was site-specifically labeled with β-CD or azobenzene to yield two complementary building blocks^CDRhuA and ^AzoRhuA. We have observed that ^CDRhuA and ^AzoRhuA co-assemble into well-defined 1D nanotubes in solution, which were characterized extensively by electron microscopy, X-ray crystallography and atomic force microscopy. Importantly, the formation of the ^CDRhuA-^AzoRhuA co-assemblies was reversible through irradiation with UV or visible light.